The problem of ghosts is certainly the main obstacle for building
a consistent QG theory. For this reason, we consider
this problem here. We shall closely follow [1]}. The interested
reader can address this book for more details, or go directly to the
original works, such as the reviews [2]} on the
Ostrogradsky instabilities, or the papers [3]} and
[4]}, [5]}, [6]} for the approach we follow to
explore and interpret the instabilities caused by massive ghosts.
